1. Field of the Invention
The present invention relates to an optical amplifier suitable for simultaneous amplification of a plurality of wavelength-division multiplexed optical signals, and also to application of this optical amplifier to an optical communication system.
2. Description of the Related Art
For construction of a future multimedia network, an optical communication system having a larger capacity is demanded. Research is being intensively conducted on an optical communication system for realization of a very large capacity, such as a time-division multiplexing (TDM) system, an optical time-division multiplexing (OTDM) system, and a wavelength-division multiplexing (WDM) system. Of these systems, the WDM system is expected to realize a flexible lightwave network by utilizing a wide gain band of an erbium doped fiber amplifier (EDFA) to perform cross-connect and add/drop in an optical region or multiplexing of different kinds of services. The WDM system is considered more advantageous than the other systems in conducting very-large-capacity transmission by using an existing 1.3 .mu.m-band zero dispersion single-mode fiber network that is now most widespread on a worldwide scale. This is due to the fact that the WDM system can realize a low transmission rate per each optical carrier and therefore set relatively large optical input power limited by permissible wavelength dispersion and the nonlinear effects of an optical fiber.
To realize the WDM system, an optical amplifier having a constant gain over a wide band is necessary. An EDFA has an amplification band (gain band) of about 1530 nm to about 1560 nm; however, the gain is not necessarily constant with respect to wavelength. That is, the gain of the EDFA changes depending on the wavelength of signal light.
As a technique for reducing the wavelength dependence of the gain of the EDFA, there has been proposed the use of a doped fiber doped with A1 and Er (C. G. Alkins et al., Electron. Lett., vol.25, pp910-911 (1989)). Further, there have been proposed other techniques such as a method of optimizing an operation point of an EDFA (M. Suyama et al., OAA'93, MB5-1 (1993)) and a method of flattening gain characteristics by using an optical filter (H. Toba et al., IEEE Photon. Technol. Lett., vol.5, No.2, pp248 (1993)). However, the above-mentioned prior art techniques for reducing the wavelength dependence of gain have a defect such that the wavelength dependence of gain becomes large when the input power of signal light or the number of WDM channels changes, because of the presence of problems inherent to each technique.